1. Field of the Invention
This invention relates to a process for preparing improved soy protein materials from denatured soy protein, and more particularly to a process for preparing improved soy protein materials from soy protein denatured by superheating treatment or alcohol extraction.
2. Description of the Prior Art
Heretofore, soy protein materials such as extracted and isolated soy proteins have been prepared from defatted soybean flake material having a high NSI (Nitrogen Solubility Index) of not less than 85% in high yields, which are usually obtained by desolventizing solvent-extracted soybean meal or flakes at a low temperature. The basic process for producing extracted soy protein comprises dispersing defatted soybean flake material having a high NSI in water at a weight ratio of flake material to water of between 1 to 5 and 1 to 15, extracting water-soluble materials from the flakes at pH 6.5 to 9.0, and separating insoluble materials from the resulting extract solution, and further comprises the steps of concentration, sterilizing and spray-drying, if necessary. Isolated protein has been prepared by adding an acid to lower the pH of the above-mentioned extract solution to about the isoelectric point of the protein, which is between about 4 and 5, collecting the precipitated protein and discarding the whey and excess liquor, and neutralizing the collected protein, and further comprises steps of concentration, sterilizing and spray-drying, if necessary. In accordance with the above processes, the yields of the extracted and isolated proteins are usually 60 to 65% and 30 to 35%, respectively.
Although superheating treatment and alcohol extraction give excellent properties to defatted soybeans, the solubility of the protein therein is unavoidably decreased to an NSI of 5 to 70%. Therefore, even if the above known processes are applied to such defatted soybeans denatured by superheating treatment and/or alcohol extraction, the yields of the extracted and isolated proteins are decreased to less than 50% and less than 20% respectively, making industrialization of such processes unattractive.
A need therefore continues to exist for a method of improving the NSI of denatured, defatted soy protein.